The present invention relates to a method for the cure of polyurethane elastomers by the use of microwave energy. Conventionally, these elastomers have been prepared by combining a suitable polyol, an isocyanate such as toluenediisocyanate and a curative such as 4,4'-methylene-bis (2-chloroaniline) (Moca) with subsequent heating to complete the curing step. Processing of such a system has posed a problem inasmuch as the reactants are readily cured in the presence of Moca to such an extent that curing commences during blending. Premature curing of the system during mixing and milling, greatly diminishes the available time for forming, increases molding problems and can result in inferior products. Obviously, it has not been feasible or possible to pre-blend such a mixture for subsequent forming operations and therefore, the shelf-life of such a system has been virtually non-existent.
It has recently been found and is taught by U.S. Pat. No. 3,755,261, that these elastomers, e.g., isocyanatoterminated polyurethane prepolymers may be cured with complexes of methylene dianiline and a halide salt such as sodium chloride. Such curing of the prepolymers is effected by heating a blend of prepolymer and complex to a temperature at which the complex liberates methylene dianiline which then acts as a conventional polyurethane curative. Below the liberation temperature, the blend is stable to polymerization or any premature reaction which is a favorable advantage over the foregoing system.
In order to cure the prepolymer/complex blend it has been conventional practice to heat it by the use of a preheated mold, hot presses or ovens employing heated fluids or air, or by infared heating. Commercially available processing units permit metering of components, subsequent mixing, vacuum degassing and a careful control over the temperature so that the prepolymer/curative blend may be heated to a range near the curing temperature in order to decrease the molding time as well as minimize molding problems.
A chief problem encountered with such a process has been that as the elastomer begins to cure, usually in the peripheral areas immediately adjacent the curing fluid or other source of heat, it becomes increasingly difficult for the heat to pass through the cured thicknesses to the inner yet uncured portions where the methylene dianiline has not yet been liberated by the heat. Continued curing time, which may ultimately produce a totally cured solid, also produces a gradient product which often possesses high internal stresses and strains which can result in premature failure, wear or other undesirable properties. Moreover, the outer surface of the product is often subjected to overcuring with accompanying deleterious effects. Lastly, due to the recognized poor heat transfer of these compositions, curing time is necessarily slow which often adversely affects the quantity of production per unit time as well as the cost of the product.
The foregoing problems are rather closely related to the curing problems experienced with rubber and other materials. In addition to the more conventional energy sources for curing, it is generally known that some rubber articles and foam materials can be cured by subjecting them to microwave energy. Because the microwaves can penetrate to the center of the substance being cured, much of the former problems of layering in the final product may be eliminated. One such process utilizes microwave heating to cure foams of compounded latexes and is described in U.S. Pat. No. 3,737,488. However, neither that patent nor any other known to us describes a process for curing polyurethane elastomers via microwave energy.